Note: You can try other complex boundary condition but we do not
recommend to do so. The main reason is that FWI is to recover
the low-frequency information of the earth model. Low-freq
means that exact absorbing is not necessarilly needed. The
result will be improved with the optimization precedure.
Furthermore, complex boundary condition (such as sponge ABC or
PML) implies more computational cost, which will degrade the
efficiency of FWI.